1. Field of the Invention
The present invention relates to a method of adjusting a servomechanism employed in an industrial machine, such as a numerically controlled machine tool (NC machine tool) or an industrial robot, and an apparatus for carrying out the method. More particularly, the present invention relates to a servo adjustment method and an apparatus which facilitate to reduce delay in response to an instructed position and to improve accuracy of position control.
2. Description of the Related Art
In a position control system which controls a moving body, such as a table, of an NC machine tool, an NC position instruction and a feedback position signal provided by a position transducer for measuring the position of a servomotor or the moving body are compared. The servomotor is controlled to make the moving body to reach a desired position so that the deviation from the instructed position may be reduced to zero.
For the purpose of carrying out circular arc cutting by the machine tool, two servomotors which drive a feed axis respectively are controlled simultaneously to allow the moving body for a circular interpolation feed motion. When rotating direction of the servomotor is reversed, the movable member cannot be followed in instant response to a instruction because of a lost motion of a driving mechanism for driving the moving body due to the rigidity and friction between the component parts.
When the moving body moves along a circular path from one quadrant to an adjacent quadrant, there takes place a response error such that an actual path of the moving body deviates and bulges our from a instructed path. Such phenomenon is called stick motion or quadrant protrusion and is one of the main cause of the deterioration of accuracy in machining. The reason why such a phenomenon takes place is presumed that the feed axis temporarily stopped due to a delay caused by the response characteristic of the velocity loop.
The response characteristic and the stability of the servo control system are affected considerably by the dynamic characteristics of power transmission mechanisms such as a feed screw and a nut for transmitting the driving power of a servomotor to the moving body, and a driven object such as a table, and a guide mechanism.
A conventional automatic control theory proves that the accuracy of a feedback control can be improved by increasing loop gain by raising frequency response. However, the servomechanism comprises component rigid members such as a power transmission mechanical system and a driven mechanical system having considerable mass, rigidity and frictional properties. In the closed loop system, oscillations occur at the resonant frequency of the system if loop gain is increased. On the other hand, following performance is deteriorated and errors increase if loop gain is reduced to give a priority to stability of the system.
In the conventional NC machine tool, various parameters affecting the characteristics of the servo control system must be set by a manual input operation for the final optimization adjustment of the servo control system. A conventional optimization adjustment procedure for the servo control system requires operators to examine the accuracy of motions of the NC machine tool. During optimization, the parameters are determined by a trial and error method while examining the accuracy of motions by using measuring instruments.
The adjustment operation of the servo control system requires the operators with considerable experiences and high skill to keep the system in the optimum condition, because important characteristics of the servo control system conflict each other; stability is deteriorated if accuracy and response characteristic are enhanced and vice versa. Particularly, recent NC machine tools and industrial robots require a control system with highly improved performance. The more the servo control system of those NC machine tools and industrial robots are highly sophisticated, the more operations for optimization become time consuming and complicated.